Ferroresonant voltage regulators having saturable and unsaturable transformers are known, where the ferroresonant circuit comprises a saturable transformer -- usually an autotransformer -- an unsaturable transformer or an unsaturable reactor or linear reactor in series with the saturable transformer and the voltage source, and a capacitor across one of the windings of the saturable transformer so as to cause a ferroresonant circuit which is tuned to the line frequency or the fundamental frequency of the output voltage from the voltage regulator. It has been a major source of concern to reduce the size and weight of ferroresonant voltage regulators, and hence the cost thereof. Balian et al U.S. Pat. No. 3,611,116 issued Oct. 5, 1971 teaches a circuit where one of the secondaries of an unsaturable transformer is connected in series-aiding with one of the windings of a saturable transformer or autotransformer, and is inclined directly in the ferroresonant circuit. The capacitor, in this case, is connected across the series connection of the secondary winding to form the ferroresonant circuit.
Other ferroresonant circuits, such as that taught in Gorbuntsov et al U.S. Pat. No. 3,662,254, issued May 9, 1972, include ferroresonant circuits where the saturable transformer winding is in series with a linear choke, but where the compensating winding is connected in series with an output circuit and capacitor. In the Gorbuntsov et al circuit, an additional compensating winding is provided in series with the capacitor to compensate variations of voltage in the capacitor circuit, resulting in a stabilized AC output. However, the output of such circuit -- while well stabilized -- may have very poor waveform -- and in any event, such output would normally be intended to be rectified to a constant voltage DC.
Thus, there are certain disadvantages which are generally found in ferroresonant circuits in AC voltage regulators; and they include the fact that the basic ferroresonant circuit yields a distorted output waveform which may be flat or dented at no-load conditions, becoming sinusoidal at full load conditions. Other disadvantages of ferroresonant circuits that have been used in the past are the fact that frequency variation and voltage changes which are caused by changes of the load and changes in ambient temperature, cannot be compensated for and may be reflected in the output voltage. Also, because prior art ferroresonant circuits operate in the high magnetizing region of the transformer core, such circuits are normally limited to relatively low KVA power ratings because of the extra heat losses which are generated in the saturable transformer and other circuit components.
On the other hand, there are certain inherent advantages in the use of ferroresonant circuits in AC voltage regulators, and they include the fact that a basic ferroresonant circuit can maintain good voltage regulation -- at least within name plate ratings -- for line voltage variations of plus or minus 15%. Ferroresonant circuits generally have a fast response time, in the order of less than a few periods of the line frequency; and they normally have very low overshoot or undershoot. Because of their nature, ferroresonant circuits have inherent current limiting and a short circuit-proof overload characteristic, and having high efficiency and inherent reliability.
As noted, however, constant voltage regulation in a basic ferroresonant circuit is achieved because it includes a saturable transformer which is designed and driven to operate in the high, saturating part of its hysteresis curve; and of course, the saturable transformer has a grain oriented steel core and may be generally referred to as a transformer having an iron core. Because the saturable transformer is designed to operate in the high saturating part of its hysteresis curve, the transformer is generally limited to low KVA power ratings because of extra heat losses which may occur.
The present invention maintains the advantages of prior, basic ferroresonant circuits including those of my prior ferroresonant circuit in the patent noted above, and provides a ferroresonant voltage regulating circuit having a basic tuned ferroresonant circuit with a synchronous switch such as a saturable reactor having a control coil, with the synchronous switch connected in shunt across at least a portion of a winding of an iron core, non-saturating transformer havng an air gap in its core. The control coil of the synchronous switch is connected in series with a three-wire semiconductor device which is connected together with suitable voltage sensing and reference circuits in the output of the voltage regulator so as to drive the synchronous switch to conductive state at no-load conditions of the voltage regulator, thereby preloading the non-saturating transformer and the basic ferroresonant circuit.
I have discovered that if the saturating transformer previously used in my earlier patent, noted above, is replaced with a transformer having an iron core with an air gap, equally good voltage regulation can be obtained -- within one percent, depending on the sensing circuits used -- while at the same time waveform distortion can be held to within one to three percent total harmonic content without the use of any additional linear reactor in the output of the ferroresonant circuit.
What is meant by a non-saturating transformer in this context is a transformer having a laminated iron core, and where the core has an air gap. The volt-ampere vs. flux density characteristic of the core and air gap of such a transformer is considerably more apparently linear than that of the core of a similarly rated and constructed saturating transformer.
By providing control of the synchronous switch which is across the non-saturating transformer, regulating AC output having closely regulated voltage and low waveform distortion can be accomplished, making the AC voltage regulator of the present invention particularly well suited for use as an AC-filter and/or waveform regulator for inverter applications.
By providing for the use of discreet components in basic ferroresonant circuit, and also for the shunt loop which basically comprises the synchronous switch, an excellent harmonic and r.f. rejection mode of a ferroresonant voltage regulator according to this invention may be achieved. Thus, the regulator may be utilized to provide a low harmonic, sinusoidal, and r.f. -- free regulated output notwithstanding the shape of the waveform or r.f. constituent of the input to the regulating circuit.
I have also discovered that certain other circuit improvements may be made to further decrease the harmonic content of the output voltage waveform and/or to further increase the response time of the ferroresonant voltage regulating circuit to changes in load conditions, where such circuit additions and improvements comprise the series connection of an additional linear reactor or cross-connected transformer with the synchronous switch so that the series connection is connected in shunt across at least a portion of a winding of the non-saturating, constant voltage transformer.